1. Field of the Invention
The present invention relates to an optical space communication apparatus for communication by means of an intensity-modulated light signal propagating in a free space.
2. Related Background Art
A conventional optical space communication apparatus is explained below with reference to FIGS. 1 and 2.
FIG. 1 is a block diagram to show an example of the conventional optical space communication apparatus, which is provided with a transmitter 500 and a multiplex modulator 700 and with a receiver 600 and a multiplex demodulator 800.
In the transmitter 500, reference numeral 501 designates an input terminal of a main signal. An electric signal input into the input terminal 501 is converted by an electro-optical converter 503 into a light signal, and the thus-converted light signal is sent through an optical system 502 in a form of light beam 506. The electro-optical converter 503 uses a light emitting device such as a laser diode (LD) and a light emitting diode (LED).
The multiplex modulator 700 is constructed as shown in FIG. 3. Sub-carriers 32a to 32d are selected at sufficient gaps to avoid mutual overlap of adjacent channels. The sub-carriers 32a to 32d are modulated by input signals of respective channels in modulators 31a to 31d, the modulated signals are multiplexed in a combiner (multiplexer) 33, and the multiplexed signal is transferred to the input terminal.
Since the optical communication may serve as a broad-band communication, signals in a broad band are normally used for the input main signal. For example, as shown in FIG. 1, in case that the multiplex modulator 700 is connected to the transmitter 500 to transmit video signals of four channels (Ch.sub.1, Ch.sub.2, Ch.sub.3, Ch.sub.4), a frequency band of the main signal ranges from several tens to several hundred MHz. (A transmission distance of the light beam signal is between several tens m and several hundred m. Although the complexity of the circuit and the optical system depends upon a distance, the fundamental construction remains unchanged.)
The light beam signal transmitted from the transmitter 500 is received by the receiver 600. In the receiver 600, the light signal is first converged by an optical system 601 onto a light receiving element in a photo-electric converter 603. An avalanche photodiode (APD), a PIN photodiode, or the like may be employed as the light receiving element. The main signal is converted by the photo-electric converter 603 into an electric signal, the electric signal is then amplified by an AGC amplifier 604, and the amplified signal is output from an output terminal 609. The receiver 600 as shown in FIG. 1 is connected to the multiplex demodulator 800. The output signal output from the output terminal 609 is separated and demodulated into four channels of video signals in the multiplex demodulator 800.
Also, in the receiver 600, a wave detection circuit 606 is used to detect a signal level of the main signal and then to monitor an intensity of reception light through a level meter 608. Further, the signal level detected is fed back to the AGC amplifier 604 to control a gain of the AGC amplifier 604, so that the level of the output signal from the output terminal 609 may be stabilized against fluctuations of the input light level. In the apparatus using the APD as the light receiving element of the photo-electric converter 603 as described above, there is such an example of the stabilization of the output signal level against fluctuations of the input light level that the signal level detected by the detection circuit 606 is fed back to a bias voltage control circuit 607 for the APD and that a bias voltage of the APD is changed depending upon the input light level to change a multiplication factor of the APD.
Some type of the transmitter 500 has a pilot signal generator 505 for generating an auxiliary signal (as will be referred to as a "pilot signal") with a frequency f.sub.1 outside the frequency band of the main signal. In this case, the pilot signal is frequency-multiplexed with the main signal in a combiner (multiplexer) 502, and the combined signal is transmitted. In a receiver for such a transmitter, a part of the received light is separated by a beam splitter 602, and is then received by a photo detector 605. The signal received by the photo detector 605 is used for example to detect a position offset of the light beam 506. In one type of the receiver such as a receiver 600A as shown in FIG. 2, a reception level of the pilot signal is detected by a pilot signal detector 610 to monitor the reception light level.
There are, however, the following problems recognized in the aforementioned conventional techniques.
The level meter 608 as shown in FIGS. 1 and 2 is necessary for position adjustment upon installation of communication line and for monitoring a reception condition in operation. The light signal received by the receiver is a direct current light intensity-modulated, in which an alternating current (AC) light signal is superimposed on a direct current (DC) light signal. An average light level, that is, the direct current light portion, is necessary for monitoring the reception light level. Because the AC light component contained in the signal changes depending upon a condition of the system. For example, the signal level changes depending upon the number of channels (multiplicity) of the video signal input from the multiplex modulator 700 in FIG. 1. The signal level also changes depending upon a length of cable, because a signal is attenuated in the cable 507 between the multiplex modulator 700 and the transmitter 500. In case of the receiver 600 as shown in FIG. 1, the detection circuit 606 detects the signal level received. In this case, the detection circuit 606, however, detects a level of the AC light signal component in the reception light level, but does not detect the reception light level itself actually necessary. In an extreme case, even if the light beam is correctly adjusted in position to be received, but if the signal is not connected to the transmitter, an indication that the light is not received is given.
In contrast, in case of the receiver 600A as shown in FIG. 2, such a problem may not occur, because a relation between the level of the pilot signal and the intensity of light is constant. However, in case of the receiver 600A, the level monitor indicates the level of the pilot signal light entering the photo detector 605, but does not reflect a level of the light entering the photo-electric converter 603, which is the light receiving element for receiving the transmitted signal. The reason is as follows. For exact indication, the photo-electric converter 603 must be completely coincident with the photo detector 605 in position relation with respect to the optical axis. It is extremely difficult that they are installed in such a complete position relation. There may be a condition that one light receiving element receives the full light incident thereinto, but the other light receiving element fails to receive the full light incident thereinto with a part of the light being off. Further, since the photo-electric converter 603 is usually different in frequency and band of the reception light from the photo detector 605, converter 603 and detector 605 employ respective elements different in size of reception plane and in kind. Thus, there could be a state in which one of the elements receives a light incident thereinto, but the other does not. Neither of the above examples can accurately monitor the light reception level accordingly.